An “ad-hoc” mobile network (ad-hoc network) is a wireless network that comprises a collection of nodes whose positions are continually changing. Unlike a regular wireless network, one can view an ad-hoc network as a network with no fixed infrastructure. For example, all the nodes function as routers and perhaps as base stations; and the mobility of the nodes causes frequent changes in network topology.
It is the varying network topology of an ad-hoc network that causes difficulty in applying routing techniques used in a conventional wireless network. In the latter, the nodes in the network are stationary and the links connecting the nodes go down infrequently. As such, it is possible to maintain the whole network topology at each node by sending topology-related information to all the nodes in the network via, what is known in the art as, “link-state,” updates. Since nodes go down infrequently—link-state updates are infrequent—and this approach works quite well in a conventional wireless network. However, in an ad-hoc network link-state changes are more frequent because of the shifting topology, thus generating many more link-state update messages throughout the ad-hoc network—and consuming valuable bandwidth in the process. Also, construction of consistent routing tables is difficult because of the delay involved in propagating link-state information.
Considering these factors, routing protocols for ad-hoc networks can be classified broadly into two categories: “table-driven” and “source initiated on-demand.” Table-driven routing protocols are similar to the above-mentioned conventional wireless routing approach, i.e., each node attempts to maintain consistent, up-to-date, routing information for all other nodes in the network. Examples of table driven routing protocols are “Destination-Sequenced-Distance-Vector” (DSDV), “Clusterhead Gateway Switch Routing” (CGSR), and the “Wireless Routing Protocol” (WRP) protocols. In contrast, source initiated on-demand routing protocols create routing information only when a source node needs a route to a given destination. Examples of source initiated on-demand routing protocols include “Ad-Hoc On-Demand Distance Vector” (AODV), “Dynamic Source Routing” (DSR), “Temporally Ordered Routing Algorithm” (TORA), and the “Zone Routing Protocol” (ZRP) protocol.
As an illustration of a source initiated on-demand protocol consider ZRP. In ZRP, each node maintains the whole network topology for a local area, or zone, around it. As such, if the node (i.e., the source node) has to send a packet to a destination address in the zone, that routing information is already available. However, if the source node has to send a packet to a destination address outside their zone, then the node initiates a query to all the nodes in the edge of its zone (i.e., edge nodes). If one of these edge nodes has the routing information for the destination address, then that routing information is passed on back to the source node.